There are so many great ones! But any of them involving Maul or Mandalore (or in some cases, both) are worth the watch.

Siege of Mandalore, but Umbara's a close second

Umbara was a good one, IMO. Also the second battle of Geonosis was awesome.

I definitely recommend the episode Sacrifice in season 6. It is my all time favorite.

Favorite clone wars arcs

The umbara arc

The siege of mandalore

Ahsoka arc s5

Darth Maul arc s5

Nightsister arc

Mortis arc

Malevolence arc

Ryloth arc

Rise of boba fett arc

Holocron arc

Obi wan undercover arc

Mon cala arc

Darth mauls return